The radio frequency identification (RFID) technique identifies an RFID tag (or an IC tag, IC card, or the like) by externally reading data such as an identifier of the tag in a non-contact manner by using an electromagnetic wave. Various application fields are possible by embedding a writable memory in the tag or combining the tag with a sensor. The RFID tags are roughly classified into an active tag (equipped with a power supply) and a passive tag (equipped with no power supply) in accordance with whether a tag has a power supply (battery).
With the advance of the downsizing technique, the passive RFID tag which operates by an electromagnetic wave supplied from an external apparatus is rapidly expanding its application fields in recent years. Well-known examples are a non-contact IC card type pass and entrance card.
It is also possible to adhere the RFID tag contained in a seal to an article, and automatically detect the existence and stored information of the RFID tag by an RFID reader installed in a location where the article is placed or near the entrance/exit of a room, thereby preventing unauthorized takeout or theft of the article. In addition, there is a plan which uses this RFID tag in efficient article management in all stages (production, distribution, and sale) of supply chain management, as the next-generation article identification technique instead of barcodes.
The transmitting power of many passive RFID tags described above is weak, so these tags can be read only in the vicinity (about a few cm to a few tens of cm) of the RFID reader. However, active RFID tags can be read from a relatively remote place (to about a few m) because the transmitting power is high.
When the RFID tags are attached to many articles, a plurality of RFID tags exist within the reading range of the RFID reader. In the conventional general system, pieces of information of a plurality of RFID tags existing within the reading range of the RFID reader can be simultaneously read, but it is impossible to determine to which object each read information belongs.
To solve this problem, Japanese Patent Laid-Open No. 10-229351 (prior art 1) discloses a system in which a specific RFID tag is irradiated with a directional optical signal, and only the RFID tag having received the optical signal transmits information on an electromagnetic wave to a reader.
Also, Japanese Patent Laid-Open No. 2000-227948 (prior art 2) discloses a system in which an RFID tag emits light, and the position of this RFID tag is detected on the basis of an image sensed by an image sensor. This method makes it possible to relate information of each RFID tag to its position information, and determine one of a plurality of RFID tags from which information is being read.
In this system, however, the position of an RFID tag is detected by using light. Therefore, if an RFID tag is located in a hidden position, it is impossible to detect the position of this RFID tag and specify it. Also, a function of receiving or emitting light must be added to an RFID tag, and this extremely complicates the structure of the RFID tag. To attach RFID tags to all articles, each individual RFID tag is desirably simple in arrangement and inexpensive.